REGTESTS: fix reg-tests/server/abnsz.vtc

REGTESTS: fix reg-tests/server/abnsz.vtc

It was proved in GH #2875 that the regtest was broken, at least for the
server-side abnsz, as the connect() was not performed using the proper
family, which results in kernel refusing to perform the call, while the
reg-test actually succeeds.

Indeed, in the test we used vtest client to connect to haproxy, which
then routed the request to another haproxy instance listening on an
abnsz socket, and this last haproxy was the one to answer the http
request.

As we only used "rxresp" in vtest client, the test succeeded with empty
responses, which was the case due to the server connection failing on the
first haproxy process.